The extremely radiation-resistant family Deinococcaceae is comprised of greater than twenty distinct species that can survive acute exposures to ionizing radiation (IR) (10 kGy), ultraviolet light (UV) (1 kJ/m2), and desiccation (years); and can grow under chronic IR (60 Gy/hour). In particular, Deinococcus radiodurans is an extremely ionizing radiation (IR) resistant bacterium that can survive exposures to gamma-radiation that exceed by a factor of one thousand the doses which are cytotoxic and lethal to mammalian cells
For extremely resistant bacteria, such as e.g., D. radiodurans, survival following high-doses of IR has been attributed to protection of proteins from oxidation during irradiation, with the result that enzymic repair systems survive and function with far greater efficiency during recovery than in sensitive bacteria, where cellular proteins are highly susceptible to carbonylation. In a report published in Science magazine (Daly et al. (2004), Accumulation of Mn(II) in Deinococcus radiodurans facilitates gamma-radiation resistance, Science 306: 925-1084), intracellular manganese(II) was implicated in facilitating radiation resistance by protecting proteins, but not DNA, during exposure to ionizing radiation; and in a second report published in PLoS Biology (Daly et al. (2007) Protein oxidation implicated as the primary determinant of bacterial radioresistance, PLoS Biology 5(4) e92), radiation resistance was positively correlated to protein protection during irradiation, mediated by a non-enzymic mechanism.
Unlike D. radiodurans, most proteins are not radiation-resistant. Similarly, most cells, whether in eukaryotes, prokaryotes or mammals (e.g. humans) are also not radiation resistant. As such, exposure to radiation is quite damaging to protein structure and/or function. For example, ionizing radiation has been shown to induce (cause) cancer in many different species of animals and in almost all parts of the human body.
In humans, significant overexposure to radiation can result in radiation poisoning, also called “radiation sickness” or a “creeping dose”. The term is generally used to refer to acute problems caused by a large dosage of radiation in a short period, though this also has occurred with long term exposure to low level radiation. The clinical name for “radiation sickness” is acute radiation syndrome as described by the CDC. A chronic radiation syndrome does exist but is very uncommon; this has been observed among workers in early radium source production sites and in the early days of the Soviet nuclear program. A short exposure can result in acute radiation syndrome; chronic radiation syndrome requires a prolonged high level of exposure.
Humans routinely encounter radiation in daily life, including radiation from electronic equipment and cell phones as well as natural background radiation. Individuals that are in close proximity of radioactive elements such as e.g. employees at a nuclear plant or members of the armed forces are particularly likely to encounter higher doses of radiation. Additionally, radiation is used in diagnostic tests such as X-rays and radiation therapy to treat cancers.
There are currently very few radioprotectors suitable for treating humans, and those which exist (e.g., amifostine) are cytotoxic and have serious side effects (e.g., loss of consciousness, fast or irregular breathing, itching, nausea and vomiting).
Given the great exposure to radiation, there is a significant need for radioprotectors that are non-toxic, preserve protein function, and in particular are suitable for human use.